7.1. System Resources

Being able to monitor the performance of your system
is essential. If system resources become to low it can cause a lot of
problems. System resources can be taken up by individual users, or by
services your system may host such as email or web pages. The ability to
know what is happening can help determine whether system upgrades are needed,
or if some services need to be moved to another machine.

7.1.1. The top command.

The most common of these commands is top.
The top will display a continually updating report
of system resource usage.

The top portion of the report lists information such as
the system time, uptime, CPU usage, physical ans swap memory usage,
and number of processes. Below that is a list of the processes sorted
by CPU utilization.

You can modify the output of top while
is is running. If you hit an i, top will no longer
display idle processes. Hit i again to see them
again. Hitting M will sort by memory usage,
S will sort by how long they processes have been
running, and P will sort by CPU usage again.

In addition to viewing options, you can also modify processes
from within the top command. You can use
u to view processes owned by a specific user,
k to kill processes, and r to
renice them.

For more in-depth information about processes you can look in
the /proc filesystem. In the /proc
filesystem you will find a series of sub-directories with numeric names.
These directories are associated with the processes ids of currently
running processes. In each directory you will find a series of files
containing information about the process.

YOU MUST TAKE EXTREME CAUTION TO NOT MODIFY THESE FILES, DOING
SO MAY CAUSE SYSTEM PROBLEMS!

7.1.2. The iostat command.

The iostat will display the current CPU load
average and disk I/O information. This is a great command to monitor
your disk I/O usage.

The iostat man page contains a detailed
explanation of what each of these columns mean.

7.1.3. The ps command

The ps will provide you a list of
processes currently running. There is a wide variety of options
that this command gives you.

A common use would be to list all processes currently running.
To do this you would use the ps -ef command.
(Screen output from this command is too large to include, the following
is only a partial output.)

The first column shows who owns the process. The second
column is the process ID. The Third column is the parent process
ID. This is the process that generated, or started, the process.
The forth column is the CPU usage (in
percent). The fifth column is the start time, of date if the process
has been running long enough. The sixth column is the tty associated
with the process, if applicable. The seventh column is the cumulitive
CPU usage (total amount of CPU time is has used while running). The
eighth column is the command itself.

With this information you can see exacly what is running on
your system and kill run-away processes, or those that are causing
problems.

7.1.4. The vmstat command

The vmstat command will provide a report
showing statistics for system processes, memory, swap,
I/O, and the CPU. These statistics are generated using data from the
last time the command was run to the present. In the case of the
command never being run, the data will be from the last reboot until
the present.

FIELD DESCRIPTIONS
Procs
r: The number of processes waiting for run time.
b: The number of processes in uninterruptable sleep.
w: The number of processes swapped out but otherwise runnable. This
field is calculated, but Linux never desperation swaps.

Memory
swpd: the amount of virtual memory used (kB).
free: the amount of idle memory (kB).
buff: the amount of memory used as buffers (kB).

IO
bi: Blocks sent to a block device (blocks/s).
bo: Blocks received from a block device (blocks/s).

System
in: The number of interrupts per second, including the clock.
cs: The number of context switches per second.

CPU
These are percentages of total CPU time.
us: user time
sy: system time
id: idle time

7.1.5. The lsof command

The lsof command will print out a list of
every file that is in use. Since Linux considers everythihng a file,
this list can be very long. However, this command
can be useful in diagnosing problems. An example of this is if you wish
to unmount a filesystem, but you are being told that it is in use. You
could use this command and grep for the name of the
filesystem to see who is using it.

Or suppose you want to see all files in use by a particular process.
To do this you would use lsof -p -processid-.

7.1.6. Finding More Utilities

To learn more about what command line tools are available, Chris
Karakas has wrote a reference guide titled GNU/Linux
Command-Line Tools Summary. It's a good resource for learning
what tools are out there and how to do a number of tasks.